Arsenal still got a chance to play and get pass this stage of the champions league and also at the same time they are at the verge of loosing it to the Porto too because if they allow their self not to in the second leg, they may stand the risk of not been able to continue with the champions league again this season and it will be a good achievement for Porto because I'm sure they wouldn't believe they would make it to this point.
There are many possibilities and scenarios that could happen in the second leg, you can see in the picture below, Arsenal is the favorite with only @1,24 odds. But we know that Porto have an advantage on the aggregate side which makes them more confident in playing defensively.
Well, with all of these and as I said before, I wouldn't bet on this match because the risk is too high.